This study is concerned: (1) with the mechanisms involved in the transmural passage from the parenchyma into the circulating blood of normal blood cells and of leukemic cells at hematopoietic sites, (2) with the organ disposition and transmural passage of leukemic cells from the peripheral blood into the parenchyma, and (3) the endocytic function of the myeloid endothelial cells. The methodology includes, transmission electron microscopy, scanning electron microscopy by imaging through secondary electrons and through the detection of backscattered electrons, the tracing of leukemic cells labeled with 3H-thymidine by means of liquid scintillation counting and also after labeling with diaminoacridines and subsequent detection by means of fluorescent microscopy. BIBLIOGRAPHIC REFERENCES: R.P. Becker and P.P.H. De Bruyn. The transmural passage of blood cells into the myeloid sinusoids and the entry of platelets into the sinusoidal circulation; a scanning electron microscope investigation. Am. J. Anat., 145: 183-206, 1976. R.P. Becker and P.P.H. De Bruyn. Backscattered electron imaging of endogenous and exogenous peroxidase activity in rat bone marrow. In: Scanning Electron Microscopy/1976/II, O. Johari and R.P. Becker, eds. Illinois Institute of Technology Research Institute, Chicago. 1976.